A handy use for this plug is to perform rudimentary cable testing. To
test an ethernet cable, for example, you can attach the loopback plug
to one end of the cable with your coupler. Then plug the other end of
the cable into an ethernet port and see if you get link. If you plug it
into a Windows 2000 or XP box, it will notify you if there is link.
With Linux you can use mii-tool or mii-diag. This works for both
straight-through and crossover cables.